Why doesn't everyone use the 'Auto Tile size' add-on?
Some tutorials were created before the addon.

People don't know the addon exists or just forget to enable it.

Old habits die hard, adjusting the tile size was an option you learned to adjust when starting to learn cycles. This would be the main reason more experienced blender users keep doing it manually.

I'm sure if you asked the tutorial creators, they will answer the same, and I doubt anyone will tell you the addon is inaccurate or no good.